The proposed research seeks to understand the development of auditory spatial perception during infancy and to delineate the acoustic features of sounds that permit infants to localize them. Whereas young infants are sensitive to the location of sound sources, their precision of localization is considerably worse than that of adults. Do infants simply have problems detecting localization cues? Or do they have difficulty mapping those cues onto space? A two-pronged attack will address these questions. Eight and 24-week-old infants will be tested in an observer-based psychophysical procedure (OPP) . They will be asked to discriminate a shift in the location of a sound, and a minimal audible angle similar to that found for adults will be estimated. This sound field procedure provides maximal localization information from sounds, but the multiplicity of acoustic cues prohibits any attribution of their relative importance. Therefore, parallel studies will present sounds via earphones to isolate the relevant auditory cues in a lateralization task. Comparisons of performance across the two tasks will begin to tell how much localization acuity is limited by detection of binaural cues. One set of studies (1 and 2) will determine whether the span of potential sound sources influences infants' sound localization and lateralization. A second series of studies will provide successively more access to binaural cues to see how that information improves infants' localization and lateralization. Transient signals will provide interaural timing cues only in their onset (Studies 3 and 4), while repeating such transients will add timing cues from the resulting signal's envelope (Studies 5 and 6). Varying the number of transients presented will directly manipulate the amount of interaural information provided (Studies 7 and 8), and a final study (9) will determine how the rate of presentation influences infants' lateralization. Together localization and lateralization procedures can highlight the limitations of infants' auditory spatial perception and indicate the degree to which adult theories of auditory perception can be generalized to infants.